Polysialic acid (PSA) is a developmental^ regulated, anti-adhesive glycan that is added to the neural cell adhesion molecule (NCAM), the most abundant of five mammalian polysialylated proteins. The presence of PSA on NCAM N-glycans negatively modulates cell adhesion and is critical for a variety of important processes including brain development, learning and memory, neuronal regeneration, and the growth and invasiveness of cancer cells. Our goal in this proposal is to understand the mechanism of protein-specific polysialylation and how the polysialyltransferases (polySTs) recognize NCAM. We have demonstrated that the first fibronectin type III repeat (FN1) of NCAM is required for the polysialylation of the N-glycans found on the adjacent immunoglobulin domain (Ig5). We have solved the crystal structure of FN1 and shown that an acidic surface patch is involved polyST recognition, and that a unique helix, which links the strands 4 and 5 of the FN1 beta sandwich structure, is critical for positioning the Ig5 N-glycans for polysialylation. We propose experiments to elucidate the mechanism of protein-specific polysialylation and to test the hypothesis that polyST-FN1 binding and an lg5-FN1 interaction are required for NCAM polysialylation. In aim I we will identifiy the FN1 residues required for polyST recognition using gain- and loss-of-polysialylation experiments. In aim II we will determine the FN1 sequences required for,and factors modulating, polyST-NCAM binding using co-immunoprecipitation and in vitro binding assays. In aim III, we will obtain the crystal structure of lg5-FN1 to determine whether these domains interact, the role of the FN1 helix in this interaction, and use binding assays and rotary shadowing electron microscopyto evaluate an alternate possibilty that the polySTs allow a transient lg5-FN1 interaction during polysialylation. Our long term goal is to understand the basis for the protein specificity of polysialylation so that we can design approaches to eliminate or enhance NCAM polysialylation during development and disease. Relevance to public health: Polysialic acid (PSA) is anti-adhesive sugar that is added specifically to the neural cell adhesion molecule, NCAM. PSA is critical for brain development, neuronal regeneration, and promotes cancer invasiveness. The goal of this project is to understand how the enzymes that add PSA recognize NCAM so that we can design approaches to diminish or enhance NCAM polysialylation and regulate its effects on cell adhesion during development and disease.